Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

Structural changes in molten high-speed steels before atomizing

Molten high-speed steel R6M5F3 has been studied by the x-ray method under different temperature and time conditions. It has been found that transformations of the microheterogeneous structure of the melt with an increase in temperature to 1750°C are associated with a change in atom packing both within microgroups of Fe ( ) and in microregions enriched in carbon (Me₆C, Me_2C MeC). The structural state of high-speed steel powders as a result of temperature-time treatment of the melt varies considerably: There is a high degree of homogeneity due to an increase in the fineness and similarity of carbide-like regions.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 8–12, July–August, 1994.     

Microstructural changes and kinetics of recrystallisation in a few dual-phase steels

Dual-phase structures were produced in three steels (designated A1, A2 and A3) using two different heat-treatment cycles which incorporate intercritical annealing at a temperature of 750°C. In general, alloy A2 contains the maximum and alloy A1 the minimum volume fraction of martensite, with alloy A3 coming in between. All the cold-rolled alloys show elongated cell-like structures and also some deformation bands. Recrystallisation anneals were carried out at 650°C and 800°C. At the lower recrystallisation temperature of 650°C, the cold-worked ferrite starts to recrystallise, whereas, at the higher temperature of 800°C, re-austenitisation of martensite and recrystallisation of cold-worked ferrite take place simultaneously. During the recrystallisation anneal strain free ferrite grains are found to nucleate at both the deformed ferrite-martensite interfaces as well as inside the deformed ferrite grains. The process of recrystallisation in all three alloys, irrespective of the initial heat-treatment as well as annealing temperature, can be described as an in-situ process. The kinetics of primary recrystallisation of ferrite in the cold-rolled dual-phase steels was analysed from the relevant microhardness data using an Avrami-type relationship. In general, the Avrami plots show straight line segments with two distinct slopes, indicating two different processes during recrystallisation. The activation energies measured from the Arrhenius plots range between 66.88 and 83.6 kJ K mole^?1 which are close to the value 84.02 kJ K mole^?1 for the diffusion of carbon in α-Fe.     

Microstructural evolution during laser cladding of M2 high-speed steel

Laser cladding of gas-atomized M2 high-speed steel on the mild steel substrate was performed using scan rates of 1 to 10 mm/s, scan line spacings of 0.1 to 0.5 mm, and powder feed rates of 1 to 10 g/min, for a given laser power of 400 W. This article presents a detailed study of the microstructural evolution during laser cladding. The effect of scan rate, scan line spacing, and powder feed rate on cooling rate can be described in terms of the cladding-layer thickness, i.e., the thinner the layer, the higher the cooling rate. The degree of metastability in the laser-clad microstructure can be understood in terms of the lattice parameter of the bcc phase. The lattice parameter of the bcc phase increased with increasing layer thickness and reached a maximum value at a thickness of 0.3 mm. Correspondingly, the microstructure varied from a cellular or dendritic structure of δ ferrite and austenite to a mixture of martensite and retained austenite. However, further increasing the layer thickness led to a decrease of both the lattice parameters of the bcc phase and the proportion of retained austenite in the martensite. This was accompanied by an increase of the amount of carbide at the prior austenitic grain boundaries and a decrease of the carbon content in the martensite and retained austenite.     

Microstructural stability during creep of Mo- or W-bearing 12Cr steels

The evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth. The important finding is that the performance of Laves-phase precipitation in the molybdenum-bearing alloy is significantly different from that in the tungsten-bearing alloy. It is generally believed that such differences in kinetics will influence creep behavior. Data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software, to determine equilibrium solute concentrations in these complex steels. The progressive depletion of Mo and W from the matrix as the particles of Laves phase evolve has been quantitatively modeled using experimental data obtained on both steels over a range of times and temperatures. The Isothermal coarsening rates of M₂₃C₆ and MX carbide particles were measured and found to occur at a constant volume fraction, in accordance with Ostwald ripening kinetics, with no significant differences in rates found between the two steels. The coarsening rates of M₂₃C₆ particles, found on subgrain boundaries, were consistent with a third-power dependence on particle radius, with an activation energy similar to that of volume diffusion. The smaller MX particles, which lay on subgrain-interior dislocation lines, were better explained by dislocation pipe diffusion, with a fifth-power dependence on particle radius and an activation energy approximately half that of volume diffusion.     

Solidification of high-speed tool steels

Gradient solidification and differential thermal analysis (DTA) experiments were used to study the process of solidification and the solidification microstructure of 11 alloys comprising the composition range of customary commercial high-speed steels (with the exception of cobalt-alloyed grades). Also included are a number of experimental high-speed steels alloyed with niobium. The results include the effects of alloy composition and cooling rate on the width of the solidification interval and on the sequence of the solidification reactions; the types of eutectics formed (austenite with M₆C, M₂C, or MC) and their volume fractions; the chemical compositions of the ledeburitic and primary carbides; and the relation between the chemistry of the carbides and that of the melt. Special attention is given to the formation and composition of heterogeneously nucleated primary MC particles and to the chemistry and stability of eutectic M₂C, which is important as a precursor to MC and M₆C in the microstructure of finished (hot-worked and heat-treated) material.     

As-cast carbides in high-speed steels

The spatial distribution and structure of as-cast carbides and the effects of W, Mo, and V content on the morphology and amount of as-cast carbides in high-speed steels were studied systematically. It was shown that increasing the Mo and decreasing the W content led to a decrease in the amount of total eutectic carbide and an increase in the MC and M₂C carbides. The eutectic morphology changed from skeletal to platelike when the content of Mo increased. The presence of V favored not only the formation of MC carbide but also the formation of M₂C carbide and reduced the formation of M₂C carbide. Increasing V led to an increase in the size of the eutectic carbides.     

Phase and structure changes during the sintering of compacts of high-speed steels obtained from powders with various rates of solidification

An investigation was made of the phase and structure transformations in compacts of high-speed steel R6M5F3 powder fractions ?630+50 μm at the temperatures of solid-phase (1160–1220°C) and liquid-phase (1240°C) sintering. The structure of the compacts was crystalline or quasicrystalline, depending on the degree of superheating and cooling rate of the melt during powder spraying. This was related to the presence of a cellular structure in particles of a critical size in the powder. Compacts of powder with the cellular structure experienced higher shrinkage during solid-phase sintering as a result of structre relaxation and recrystallization. The driving force of these processes is the change in chemical potential arising from the decomposition of highly supersaturated metastable solid solutions, and the excess grain-boundary energy of the quasicrystalline matrix structure.     

Microstructural changes during creep of a high chromium steel

This article paper deals with creep deformation of a high-chromium steel, X-20. Creep tests were performed at three temperatures, 500 °, 550 °, and 600 °, under various loads. Some of the tests terminated in rupture, while others were interrupted and examined by optical micro-scopy and scanning electron microscopy (SEM). It was found that at the onset of tertiary creep, the creep rate begins to decrease for a certain period of time and a “step” appears on the creep curve. Study of the microstructure revealed that at the beginning of the step, the martensite needles start to rotate toward the tensile axis, and at the end of the step most of the needles are parallel to this direction. Hot-rolled pieces of the steel confirmed this observation; after rolling to a reduction of 35 pct, all the martensite needles lie parallel to the rolling direction and, therefore, the step was not observed. We offer an explanation for the step on the creep curve based on the rotation of the martensite.     

Microstructural changes occurring during the gaseous reduction of magnetite

The reduction of dense magnetite samples in H₂/H₂O and H₂/N₂ gas mixtures between temperatures of 723 and 1373 K has been investigated. A detailed study of partially reduced samples using conventional metallographic and scanning electron microscopy (SEM) techniques has enabled the conditions for the formation of a number of different product microstructures to be clearly defined. The reaction mechanisms involved in various continuous and discontinuous growth processes are discussed. Formerly with the University of Queensland.     

Grain size of high-speed tool steels

Influence of the carbide population on austenite formation during hardening of high speed tool steels has been investigated. It was established that austenite grain size varies directly with the mean primary carbide size or interparticle spacing. The type of annealing treatment—temper annealing or transformation annealing—given prior to hardening has an additional effect on the subsequent austenite grain size. This effect is related to the characteristics of the secondary carbide population. Austenite grain refinement was found to significantly improve the performance of high speed steel tools operating under intermittent cutting conditions. The observed tool wear during intermittent cutting is explained in terms of a combination of mild wear and “microspalling” mechanism at the cutting edge.     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Microstructural changes in a 12% chromium steel during creep

A quantitative metallographic study was performed using transmission electron microscopy (TEM) to describe the microstructural changes in a 12% chromium steel (X 20 CrMoV 12 1) during creep at 650°C. The creep experiments were conducted under constant load conditions corresponding to initial stresses of 175 and 80 N/mm². The heat treatment for this steel consists of austenitizing followed by tempering which results in a high density of free dislocations within small elongated subgrains with carbides on or very near some of the subgrain boundaries. During creep, the mean subgrain size increases for both the high and low stress levels. Carbide particle coarsening is observed for the low stress level. These processes result in a softening of the microstructure during creep deformation.     

New high-nitrogen corrosion-resistant tool and high-speed steels

The methods of nitriding of metals in traditional technological via ingot processes are briefly described, and their disadvantages are noted. The variations of nitriding powder metallurgy are given, and their advantages are discussed. The world experience in the production of nickel-free, corrosion-resistant steel with 1% N by powder metallurgy is analyzed, and experience in the production and operation of powdered high-nitrogen tool and high-speed steels is considered.     

高速钢粉末真空高温处理时组织结构之变化

研究了水雾化M 2和M 4高速钢粉末在 7× 10 -2 Pa真空下加热后组织结构的变化。当粉末加热至固相线以下 15～ 2 0℃ ,即 12 2 0～ 12 2 5℃时 ,M6C及MC型碳化物都由 1～ 2 μm长大到 3～ 4 μm ;当粉末加热至固相线以上时 ,碳化物尤其是M6C型碳化物颗粒快速长大 ,并且有不均匀长大现象 ;当温度升至 12 5 0℃后 ,贫碳型的M4C3 转变为MC型碳化物 ,其中大部分Fe被V所取代 ;当进一步加热至 12 6 0℃以上时 ,M6C型碳化物将转变为M2 C碳化物。经真空脱氧后高速钢粉末的氧含量降低至 10 0× 10 -6以下。